1. Field of the Invention
The present invention relates to a fluid piston-cylinder unit in which a piston, movably carried by a piston rod, divides a cylinder into two working chambers, and a sealing valve closes a fluid flow path across the piston when the piston rod is stationary.
2. The Prior Art
DE 3,301,544 A1 discloses a piston-cylinder unit comprising a piston, which is carried for axial movement by a piston rod, and a cylinder divided into two working chambers by the piston. A sealing valve seals off a fluid flow connection between the two working chambers when the piston rod is in a stationary operating condition. The sealing valve includes a valve element consisting of an axially movable sealing ring in the form of a packing washer in conjunction with a valve seat. The problem in this piston-cylinder unit is that very precise finishing is required to achieve tightness of the sealing valve. A spring, which prestresses the packing washer on the valve seat, is arranged at the back of the washer. The spring is designed with a ground end coil. As a result, under some circumstances the force of the spring on the valve washer is not uniform, so that the washer may become slightly distorted. The resulting fluid leakage is to be avoided. Structural difficulties ensue from the fact that, on the one hand, a considerable annular clearance (see reference numeral 13) and, in addition, a valve opening (reference numeral 18) must be provided for satisfactory operation. The small radial space available limits the possible cross sectional area of the fluid connection between the working chambers.
DE 4,431,626 A1 discloses a piston-cylinder unit having a sealing valve wherein unlike large pressure-impacted surfaces, in conjunction with a ram pressure, hold the sealing valve in an open position. The logic of this valve design lies in that the sealing valve remains in the open position with the application of as small a force as possible. Once the sealing valve is opened, a pressurized piston-cylinder unit with gas prestress will continue to travel out until the unit is brought to a standstill by hand and a pressure equalization that releases the ram pressure is produced. The basic idea of this sealing valve is quite different from that of DE 3,301,544.
DE 3,301,266 A1 describes a gas spring with hydraulic or pneumatic attenuation as a slide-in safety device in the event of loss of pressure. The gas spring has a piston arranged on a piston rod, the piston on the side opposite the piston rod being provided with a piston bore. The piston bore contains a spring chamber, in which a helical spring or a pressurizing gas is arranged. The spring or gas biases a valve pin towards sealing engagement with a valve seat. In normal operation, the operating pressure of the gas spring is higher than the spring force of the helical spring so that the sealing valve is normally open. If the operating pressure in the gas spring drops, the helical spring seals the valve opening and thus prevents sudden retraction of the piston rod.
DE 2,653,552 C2 concerns a gas spring which is designed as a lifting aid for opening and holding tailgates and trunk lids of motor vehicles. The gas spring includes a cylinder filled with pressurized gas, a piston rod arranged displaceable coaxially in the cylinder and extending in a sealed manner out of one end of the cylinder. The piston rod carries a guide piston which sealingly engages the inner wall of the cylinder and which includes a pressure-dependent valve for holding the piston stationary against a specified push-in force exceeding the weight of the tailgate or of the trunk lid. The guide piston has at least one fluid flow channel for connecting the two working chambers. When the specified push-in force is exceeded, the valve opens to permit fluid flow through the channel, thereby permitting inward travel of the piston rod.